1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus and a method for purifying an exhaust gas.
2. Discussion of the Background
Since emphasis has been put on environmental problems in recent years, an engine with good fuel economy and a small environmental impact is required in the auto industry. Advantageously, the diesel engine is superior in fuel economy to the gasoline engine. On the other hand, since particulate matters (hereinafter, also referred to as PMs), such as soot, are generated, it is necessary to purify PMs in exhaust gas by providing an exhaust gas purifying apparatus.
The gasoline engine advantageously has less emission of particulates such as soot compared with the diesel engine. Therefore, in general, it is presumably not necessary to provide an exhaust gas purifying apparatus for purifying PMs. On the other hand, disadvantageously, the gasoline engine is inferior in fuel economy to the diesel engine.
Since automobile buyers tend to make much of fuel economy of automobiles in recent years, the number of mounted gasoline direct injection engines (GDI) excellent in fuel economy among gasoline engines is expected to increase. However, since the exhaust gas emitted from the gasoline direct injection engines contain a small amount of PMs, it seems necessary to install an exhaust gas purifying apparatus and purify PMs in the exhaust gas.
The exhaust gas purifying apparatus used for purifying an exhaust gas emitted from a diesel engine is manufactured by installing, in a metal casing (casing), a filter made of materials such as ceramics. An exhaust gas can be purified by introducing the exhaust gas into the exhaust gas purifying apparatus from the gas inlet side thereof, passing the exhaust gas through the filter, and discharging the exhaust gas from the gas outlet side thereof.
As a ceramic filter for purifying PMs in an exhaust gas, U.S. Pat. No. 4,417,908 discloses a honeycomb filter which has a honeycomb structure and whose aperture area on the gas inlet side is larger than the aperture area on the gas outlet side.
WO2004/024293 A1 discloses a honeycomb filter whose aperture area on the gas inlet side is larger than the aperture area on the gas outlet side and which is formed by combining a plurality of pillar-shaped porous ceramics members. WO2005/002709 A1 discloses a honeycomb filter whose aperture area on the gas inlet side is larger than the aperture area on the gas outlet side and whose pore-size distribution is adjusted to a predetermined range.
The contents of U.S. Pat. No. 4,417,908, WO2004/024293 A1 and WO2005/002709 A1 are incorporated herein by reference in their entirety.